Conventionally, as a solid-state imaging element, there has been a sensor element using a single crystalline silicon substrate and a sensor element using an amorphous silicon film.
A characteristic of the sensor element using a single crystalline silicon substrate is that high output is achieved by forming an output amplifier circuit over the single crystalline silicon substrate and integrating it with the sensor element. However, the form of a completed packaged unit is not slim since a correction filter for wavelength sensitivity is required. In addition, the sensor element using a single crystalline silicon substrate has a problem in that the range of variations is increased by using a filter or the like.
On the other hand, a characteristic of the sensor element using an amorphous silicon film is in that a correction filter such as an infrared light cut filter is not required since the wavelength sensitivity is close to that of human eyes, while there is a limit since an output value of the sensor element is not amplified. In addition, it is easily affected by noise of other signals or the like since the output value of the sensor element is small. The output value of the sensor element depends on the absolute amount of the sensor element (e.g., area, thickness, etc.). Therefore, in order to increase the output value of the sensor element using an amorphous silicon film, the area thereof was required to be increased accordingly.
It is possible to use by amplifying the output of the sensor element using an amorphous film by providing an operational amplifier externally to the sensor element using an amorphous film. In this case, however, there was another problem that the number of external parts is increased and the sensor circuit becomes large.
The photosensitivity of the sensor element using an amorphous film is lower than half of that of the sensor element using a single crystalline silicon substrate. Therefore, there is a problem that a display device requiring a large area such as a liquid crystal projector is further easily affected by noise. Since a shield for covering a wire and the like are necessary in order to use the sensor element using an amorphous film in the large display device, cost of the display device increased.
The present inventor has suggested Patent Documents 1 to 4 concerning a semiconductor device having a sensor element and a circuit comprising a TFT over a glass substrate.
[Patent Document 1] Japanese Patent Laid-Open No. Hei6-275808
[Patent Document 2] Japanese Patent Laid-Open No. 2001-320547
[Patent Document 3] Japanese Patent Laid-Open No. 2002-62856
[Patent Document 4] Japanese Patent Laid-Open No. 2002-176162